Bobby and Billy can complete a job in 28 days and 20 days, respectively. They work together for 5 days, after which Billy leaves the job. In how many days did Bobby alone complete the rest of the job?

Let's assume the total work is 140 units.

Bobby and Billy can complete a job in 28 days and 20 days, respectively.

Efficiency of Bobby = $$\frac{140}{28}$$ = 5 units/day

Efficiency of Billy = $$\frac{140}{20}$$ = 7 units/day

They work together for 5 days.

Work done by both of them together in 5 days = $$5\times\ \left(5+7\right)$$

= $$5\times12$$

= 60 units

Remaining work = 140-60 = 80 units

Time taken by Bobby alone to complete the rest of the job = $$\frac{80}{5}$$

= 16 days